1. Field of the Invention
The present invention relates to RF and microwave filters, and more particularly to simplifying the filter design and prototype processes.
2. Description of the Related Art
Presently, RF and microwave filters (RFMF) are used extensively in most communication devices, radar and RF/microwave systems. They are used to create the desired RF or microwave output signal-free of unwanted spurious signals and with the proper output characteristics. RF/microwave telecommunication equipment manufacturers use millions of these filter per year. These filters are used in cellular basestations, satellite communication systems and microwave communication links to name a few typical applications. RFMF components are either made internally by the equipment manufacturer or procured externally. Most of the time these filters are procured because the required filter specifications are often difficult to manufacture, and thus many companies specialize in making RFMF designs. Such filters range in frequency from xcx9c5 MHz to 100 GHz, usually in the 200 MHz to 4 GHz range. Some companies focus a great deal into military systems while, others focus on commercial products. Many different types of filters are made by these companies including dielectric filters (using conductivity coated ceramic blocks), LC filters, comb filters, notch filters, helical filters, coupled cavity filters and the like. Most companies make custom filters but have a catalog of standard filters. Some companies, but not many, have many standard filters. Most companies and their distributors do not stock standard filters.
Engineers using filters usually write their own specifications so that a company can submit a design proposal. Some companies have software to help engineers specify and define filters. If the engineer likes the proposal they request or buy samples from the manufacturers they prefer. This process generally takes four to twelve weeks. When the engineer gets the RFMF, he tests it and sometimes makes changes to the requirement and the process continues, thus sometimes the system requirements change as the design progresses. Spurious signals become apparent and they have to be reduced, e.g., by RF emission testing (per FCC criteria) which may require different filter characteristics, etc. Accordingly the process may require about one to six months to complete. If the filters, however, are not too difficult to make and the cost is a major consideration the filters are sometimes made internally using standard inductors and capacitors, or by on board techniques such as microstrip coupled lines. Some companies sell variable filters which can tune over a wide range of frequencies, however these filters are expensive, large, connectorized, and thus for most situations cannot be used in prototype systems.
There are numerous shortcomings associated with existing filter design practices, such as design time, lack of flexibility, difficulty in communicating needs, and various difficulties associated with simulating and building prototypes. First, as discussed above this process can take up to six months or more to build and test a desired filter design. Alternatively, the circuit designer may use commercially available parts, but must then contend with the attendant lack of flexibility and availability of a particular filter characteristic. Thus the engineer must modify their circuit design to accommodate the use of the limited number of readily available filters. To this end, one must take what is given and cannot change many times because of the cost and time constraints associated with standard and custom filters.
Secondly, many times difficulty arises in communicating the engineers exact filter requirements because the systems are often so complex that it is difficult to communicate every specification which is required. For example, the filter manufacturing company might build the filter for a 50 ohm load but what is actually needed is a different impedance. Often the engineer does not know exactly what he really wants until the system is put together. As a result the filter maybe incorrectly specified.
Furthermore, difficulty occurs in simulating a circuit or system because of the lack of exact information on the filter. Many other components such as amplifiers, attenuators, and switches are well characterized by the manufacturers and their S-parameters can be put into computer programs that simulate the circuit or system accurately. Filters also present a design problem because many times the engineer does not know the exact response or impedance requirement until the engineer receives the actual part from which components are characterized to extract the S-parameters. Some system simulators only require the passband, rejection and group delay of the filter, but more detailed circuit simulators require S-parameters or an equivalent circuit.
Finally, filters are often the rate determining step when building a RF/microwave system and many times present the most significant difficulty to building a the system quickly. Other components such as amplifiers, attenuators, switches, and mixers are broadband such that standard product will be available in short notice from many manufacturers and distributors. Filters are generally not broadband and are by definition frequency specific. With the exception of some standard telecommunications frequency filters, most are typically not held in stock because of their specialized nature. Many times engineers desire to modify a standard filter""s characteristics such as bandwidth, rejection, ripple, impedance, etc.
Numerous problems are associated with building experimental high frequency filters on test boards. They include a lack of performance due to low Q components and board type restrictions, tuning requirements, as well as the time required to build and test the filter design. Generally a test board must be created, components must be characterized at required frequencies, and finally the filter must then be tested and tuned.
It is an object of the invention to overcome the existing filter problems of the prior art.
It is an object of the present invention to provide circuits and methods of making high frequency filters which may be designed and assembled in minutes instead of months.
It is another object of the invention to provide filters which can be optimized and well characterized before they are ever built.
It is yet another object of the present invention to provide filters that can be optimized in the real system for maximum performance and control.
It is a further object of the invention to provide cost effective filter designs through the use of readily available competitive components.
It is a still further object of the invention to provide for manufacture with enhanced turn around time and communication of design specifications that may use filter design software which specifies the basic components required to build the specific high frequency filter. Thus allowing the user to build prototype filters that may be provided as a sample to a filter manufacturer or given in the form of specifications of the existing filter.
In a described embodiment, a kit for assembling a high frequency filter includes a filter case having side walls, a generally open proximal end and a generally closed distal end. A partition within said filter case separates the inside of the filter case into at least a first cavity and a second cavity, the partition having an aperture for coupling the first and second cavities. A first helical resonator coil is disposed inside the first cavity of the filter case extending from the proximal end towards the distal end of the filter case, and a second helical resonator coil is disposed inside the second cavity of the filter case extending from the proximal end towards the distal end of the filter case.
A first tap coil is then provided as being connectable in series with the first helical resonator coil at the proximal end of the filter case, the series connection between the first helical resonator coil and the first tap coil providing an input tap for coupling electrical signals to the high frequency filter. A second tap coil is further connectable in series with the second helical resonator coil at the proximal end of the filter case, the series connection between the second helical resonator coil and the second tap coil providing an output tap for coupling electrical signals from the high frequency filter. A removable tap housing is provided for supporting the first tap coil at the proximal end of the filter case.
A method of assembling the high frequency filter thus provides a first coil for resonating first electrical signals, and a second coil for resonating second electrical signals. The first and the second coils are enclosed between a generally open proximal end and a generally closed distal end. Partitioning of the enclosed first and second coils provides a first cavity and a second cavity respectively. The first coil is disposed inside the first cavity extending from the proximal end towards the distal end, and the second coil is disposed inside the second cavity extending from the proximal end towards the distal end of the enclosure. A removable signal coupler provides coupling of electrical signals into the first coil, with the coupling tap being supported by a housing at the proximal end.
Briefly summarized, the present invention relates to filters and methods wherein resonating first and second electrical circuits are enclosed between proximal and distal ends of a filter case. Partitioning the inside of the enclosed resonant circuits may be performed by a user to form at least a first cavity and a second cavity. The first resonating circuit is then disposed inside the first cavity of the filter case extending from the proximal end towards the distal end, and the second resonating circuit is disposed inside the second cavity also extending from the proximal end towards the distal end. Electrical signals are coupled into the resonating circuits by an encased signal coupler which is removably mounted by a coupling housing for supporting the signal coupler at the proximal end of the filter case for positioning in the vicinity of the resonating circuits.
These and other objects and advantages are realized by high frequency filter design techniques for simplifying the overall specification and prototype processes. The appended claims set forth the features of the present invention with particularity. The invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings.